It is common practice in the preparation of particulate zinc oxides for incorporation in various organic media, such as rubbers and plastics, to treat such particulate zinc oxides with an organic compound such as, for instance, propionic acid, in order to make the surface of such zinc oxide hydrophobic and thus improve the dispersion time of the resultant zinc oxide into the rubber or plastic organic media.
As a result of treating such zinc oxides to improve their dispersion characteristics, one generally obtains a particulate zinc oxide which is extremely dusty and which presents a significant disadvantage in handling in that the increased dustiness, particularly of propionic acid treated zinc oxide, creates environmental pollution problems and, in addition, presents process control problems and results in an unpleasant working environment.
It is, therefore, of primary importance that the certain treated zinc oxides, and in particular propionic acid treated zinc oxides, be further modified to reduce their dusting index, which is a measure of their dustiness characteristics, while, at the same time, maintaining the improved dispersion time characteristics of such materials, which has been achieved via the initial treatment of such materials.
Various processes have been suggested to reduce the dustiness of propionic acid treated zinc oxide materials, all of which have met with limited acceptance either due to their minimal effectiveness or to the adverse impact of such processes upon the previously enhanced dispersion time characteristics of the resultant treated materials.
For example, the addition of light processing oils such as Circo oil, a product of the Sun Oil Company, can help to reduce the dusting index, however, the reduction obtained is not considered sufficient for most applications.
In addition, various processes involving pelletizing and/or densification of the treated zinc oxide material have been suggested and, while successful in reducing the dusting index, result in a material whose dispersion time characteristics are adversely affected.
Exemplary of various processes which are available in the prior art are the following:
U.S. Pat. No. 2,251,871 to Gamble et al. which issued on Aug. 5, 1941, relates to the manufacture of zinc oxide pigments in which the zinc oxide particles are surface coated with a zinc phosphate. In order to achieve this surface coating the zinc oxide is heated with an ester of phosphoric acid, such as aryl or alkyl phosphates, in such a fashion that these phosphates are decomposed while in contact with the surface of zinc oxide particle with the resultant formation of a phosphate coating. The express purpose of such treatment is to reduce the reactivity of the zinc oxide particles with various acidic ingredients in paint vehicles and to increase resistance to metallic staining.
U.S. Pat. No. 2,333,367 to Eide et al. which issued on Nov. 2, 1943, discloses the use of phosphorus oxides in combination with various alkyd resins to reduce the hazing of zinc oxide pigments.
U.S. Pat. No. 3,003,965 to Troelstra et al. which issued on Oct. 10, 1961 relates to methods of distributing powders in organic liquids in which the powdered material to be distributed is treated with a derivative of phosphoric acid in which a phosphorus atom carries at least one acid function and at least one alkyl-, aryl- or aralkyl group on an interposed oxygen atom.
U.S. Pat. No. 3,290,164 to Ferrigno et al. which issued on Dec. 6, 1966 relates to kaolin clay which is coated with a halogenated organic phosphorus compound and to improved polyester resin compositions formulated using the novel coated clays.
U.S. Pat. No. 3,545,994 to Lott, Jr. et al. which issued on Dec. 8, 1970 relates to the treatment of pigmentary metal oxides via coating in an aqueous slurry with a hydrous metal oxide by adjusting the slurry's pH with a weak acid or weak base which is generated in situ, requiring the use of a water soluble hydrolyzable compound which will hydrolyze to generate the weak acid in situ.
U.S. Pat. No. 3,649,321 to Durrant et al. which issued on Mar. 14, 1972 relates to titanium dioxide particles having improved resistance to dusting prepared by providing a coating on the particles of a mixture of at least one ester and at least one silicone.
U.S. Pat. No. 3,808,022 to Twist et al. which issued on Apr. 20, 1974 teaches pigmentary titanium dioxide which has been coated with a propane diol in an amount of from 0.05 to 3 weight percent, the particles of which may also be optionally coated with one or more hydrous metal oxides prior to coating with propane diol.
None of the foregoing prior art teachings suggest the method which is the subject of the present application which involves the treatment of a particulate zinc oxide material, which has been previously treated to improve dispersion time in organic media, in order to improve the dusting index thereof.
It is, therefore, an object of the present invention to provide a method for the preparation of treated particulate zinc oxides, which zinc oxides have been previously treated to enhance their dispersion time characteristics in various organic media, in order to improve the dusting index of such zinc oxide materials, while, at the same time, maintaining the enhanced dispersion time characteristics achieved via the initial treatment.
It is a further object of the present invention to provide a method for the preparation of a particulate zinc oxide material which will be commercially acceptable for incorporation into various organic media, such as rubbers and plastics, comprising the initial treatment of such zinc oxide material with an organic compound, such as propionic acid, to improve the dispersion time thereof in an organic media, and the subsequent treatment of such zinc oxide material with a composition selected from the group comprising trialkyl phosphates and tricresyl phosphate to improve the dusting index of the resultant material.
It is yet another object of the present invention to provide for a treatment for reducing the dusting of a particulate zinc oxide material, which has been previously treated with propionic acid, for use in incorporation in rubbers and plastics, comprising treating said particulate zinc oxide material with a composition selected from the group comprising trialkyl phosphates, tricresyl phosphate and mixtures thereof in an amount sufficient to reduce the dusting index thereof.
These and other objects of this invention will become apparent in the following description of the invention.